1. Field of the Invention
The present invention concerns wireless communication, and more specifically, to a method and apparatus for performing a random access procedure in a wireless communication system.
2. Related Art
3GPP (3rd Generation Partnership Project) TS (Technical Specification) Release 8-based LTE (long term evolution) is a promising, next generation mobile communication standard.
As set forth in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8),” physical channels, in 3GPP LTE, may be categorized into downlink channels such as PDSCH (Physical Downlink Shared Channel) and PDCCH (Physical Downlink Control Channel) and uplink channels such as PUSCH (Physical Uplink Shared Channel)  PUCCH (Physical Uplink Control Channel).
In order to reduce interference in uplink transmission between terminals, it is critical for a base station to maintain the terminals' uplink time alignment. A terminal may be positioned in some area of a cell, and the arrival time of an uplink signal transmitted from a terminal to a base station may vary depending on the position of each terminal. The arrival time of a terminal positioned at a cell edge is longer than that of a terminal positioned at a center of the cell. In contrast, the arrival time of the terminal positioned at the center of the cell is shorter than that of the terminal positioned at the cell edge.
A base station needs to perform scheduling that allows uplink signals transmitted from terminals in a cell to be received within a boundary at every time so as to reduce interference between terminals. The base station should properly adjust transmission timing of each terminal, and such adjustment is referred to as uplink time alignment. A random access procedure is one of processes for maintaining uplink time alignment.
Recently, use of a plurality of serving cells has been introduced to offer a higher data rate. The existing random access procedure has been designed considering only one serving cell.